Re: Why does Microsoft want this?
I think, officially, it's so that iOS apps can more easily be ported to Windows Phone and to mainline Windows. So frameworks like this are reproduced identically as they're just processing things, intended to allow work to get done. However Microsoft intends to provide distinct UI libraries that merely conform to an iOS coder's expectations on patterns and idioms. So you still end up writing a native Windows application, you just do it in a language and using a language binding that was not previously supported and which more easily facilitates conversion. You definitely don't just hit build and get some pretend-iOS sandbox that kind of connects to the Microsoft equivalents of some things in some places to some extent.